Fuel cell

ABSTRACT

In a fuel cell comprising an electrolyte-retaining matrix interposed between a pair of gas-diffusion electrodes, the matrix comprises particles or fibers of a substance unreactive with phosphoric acid and having electron-insulating properties and an inorganic binder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phosphoric acid-type fuel cellcomprising a pair of gas-diffusion electrodes and a porous matrixretaining phosphoric acid as an electrolyte (hereinafter referred to asphosphoric acid electrolyte-retaining matrix).

Particularly, the present invention relates to an electrolyte-retainingmatrix and a process for producing the matrix.

2. Description of the Prior Art

As phosphoric acid electrolyte-retaining matrix, there has been used aphenolic resin fiber cloth or nonwoven fabric or a mixture of siliconcarbide powder and polytetrafluoroethylene (an organic binder).

These matrices are described in detail in the specification of U.S. Pat.No. 4,017,664. According to the disclosure of the specification of U.S.Pat. No. 4,017,664, phosphoric acid reacts with phenolic resin at atemperature of above 250° F. (about 121° C.) in the phenolic resin-typematrix. By this reaction, molecules which are adsorbed on the electrodecatalyst causing deterioration of the activity of the catalyst areformed, whereby the performance of the fuel cell is deteriorated. Forpreventing the reaction of phosphoric acid with the phenolic resin, itis effective to lower the operating temperature of the cell. However,another problem of serious reduction of output of the cell is posed bythe lowering of the working temperature. When a mixture of siliconcarbide powder with polytetrafluoroethylene (organic binder) is used asa matrix material, the operating temperature of the fuel cell can beelevated to about 190°-200° C. However, organic binders are generallywater-repellent and particularly fluorine resins such aspolytetrafluoroethylene has a high water-repellency and no affinity withphosphoric acid. Therefore, the electrolyte-retaining matrix has only apoor phosphoric acid-retaining capacity and the phosphoricacid-retaining capacity thereof is reduced gradually.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a phosphoric acid-typefuel cell having a novel electrolyte-retaining matrix having aphosphoric acid-retaining power higher than that of anelectrolyte-retaining matrix comprising a mixture of silicon carbidepowder and polytetrafluoroethylene.

Another object of the present invention is to provide a process forproducing the above-mentioned, novel electrolyte-retaining matrix.

Still another object of the present invention is to provide a processfor producing an electrolyte-retaining matrix which can be stored easilybefore the incorporation in a fuel cell.

The fuel cell of the present invention comprises a pair of gas-diffusionelectrodes and a porous matrix for retaining phosphoric acid electrolyteand is characterized in that the matrix consists essentially of asubstance unreactive with phosphoric acid and having electron insulatingproperties and an inorganic binder.

According to the present invention, the above-mentioned matrix can beproduced by following process (1) or (2):

(1) a process wherein a mixture of a powdery or fibrous substance whichis unreactive with phosphoric acid and which has electron-insulatingproperties and a phosphate-forming inorganic material is prepared, thenphosphoric acid is added to the mixture in an amount larger than thatrequired for forming a phosphate and the whole is heated to form thephosphate, or

(2) a process wherein a mixture of a powdery or fibrous substance whichis unreactive with phosphoric acid and which has electron-insulatingproperties and a phosphate-forming inorganic material is prepared, thenphosphoric acid is added to the mixture in an amount required forforming a phosphate and the whole is heated to form the phosphate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the structure of a single cell ofphosphoric acid-type fuel cell.

FIG. 2 shows a model of an electrolyte-retaining matrix of the presentinvention.

FIG. 3 is a graph showing relationships between voltage and currentdensity in a fuel cell shown in an example of the present invention anda fuel cell used in the prior art.

FIG. 4 is a graph showing relationships between voltage and operationtime of a fuel cell shown in another example of the present inventionand a fuel cell used in the prior art.

FIG. 5 is a graph showing relationships between voltage and working timeof a fuel cell shown in still another example of the present inventionand a fuel cell used in the prior art.

FIG. 6 is a graph showing a relationships between voltage and operationtime of a fuel cell shown in a further example of the present inventionand a fuel cell used in the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The substance unreactive with phosphoric acid and havingelectrone-insulating properties will be referred to as "phosphoricacid-resistant insulating substance".

The amount of the phosphoric acid-resistant insulating material in thephosphoric acid-type fuel cell of the present invention is 30-90 wt. %,particularly 50-70 wt. %, based on the electrolyte-retaining matrix. Thebalance is substantially an inorganic binder. If the amount of thephosphoric acid-resistant insulating substance is less than 30 wt. %,the volume of the pores retaining phosphoric acid is reduced and thephosphoric acid-retaining capacity becomes insufficient. If the amountthereof is more than 90 wt. %, the binding power of the phosphoricacid-resistant insulating substance becomes unsatisfactory. Thephosphoric acid-resistant insulating substance is preferably in granularor fibrous form. The average particle diameter is preferably 0.1-10 μmand the length of the fiber is desirably 0.01-0.2 mm.

The amount of the inorganic binder is in the range of 10 wt. % to 70 wt.%. The preferred amount is 30-50 wt. %. If the amount of the inorganicbinder is less than 10 wt. %, the power of binding the phosphoricacid-resistant insulating substance is poor and, on the other hand, ifsaid amount is more than 50 wt. %, the pores are covered by theinorganic binder to reduce the effective volume thereof.

By binding the phosphoric acid-resistant insulating substance by meansof the inorganic binder, the following superior effects are obtained ascompared with effects obtained by using polytetrafluoroethylene (anorganic binder):

(i) A high phosphoric acid-retaining capacity can be obtained, since theinorganic binder has a high affinity with phosphoric acid.

(ii) The inorganic binder is stable against heat and, therefore, it isnot denatured even if it is used for a long period of time in phosphoricacid at a high temperature.

Preferred inorganic binders are metal phosphates. The metal phosphateshave a quite high affinity with phosphoric acid and exhibit a strongbinding power for the phosphoric acid-resistant insulating substancelike polytetrafluoroethylene. As the metal phosphate, there is usedpreferably at least one compound selected from the group consisting ofzirconium, tin, titanium, silicon and aluminum phosphates. Among them,zirconium phosphate is particularly preferred. These metal phosphateshave a characteristic feature that they are stable in phosphoric acid ata high temperature and they are not denatured while they are used for along period of time. Among them, zirconium phosphate can be preparedeasily and have a high stability at a high temperature.

The phosphoric acid-resistant insulating substance has as strong aspossible properties which are described in lines 23-36, column 1 of thespecification of U.S. Pat. No. 4,017,664 in addition to the unreactivitywith phosphoric acid and the electron-insulating properties. Forrealizing the electron-insulating properties, this substance hasdesirably an electric resistance of at least 10⁷ ωcm. Inorganiccompounds such as oxides, carbides and nitrides have these properties.As preferred inorganic substances, there may be mentioned oxides such ascompound oxides mainly comprising zirconium, e.g. zircon and tantalumpentoxide, carbides such as silicon carbide and tantalum carbide andnitrides such as boron nitride. Among them, silicon carbide is mostpreferred, since it is most inexpensive and available in abundance.

A substance which forms the phosphate upon the reaction with phosphoricacid is mixed with the phosphoric acid-resistant insulating substanceand then phosphoric acid is added thereto to effect thephosphate-forming reaction. A higher binding power for the phosphoricacid-resistant insulating substance is obtained by the latter processthan that obtained by the process wherein a metal phosphate is used fromthe initial stage.

The substance which forms a phosphate upon the reaction with phosphoricacid will be referred to as "phosphate-forming substance". Thissubstance is at least one member selected from the group consisting ofphosphate-forming metals, metal oxides and metal salt solutions.

When the metal oxide is used, the metal oxide MO_(x) reacts withphosphoric acid X.H₃ PO₄ to form a phosphate M(HPO₄)_(x) which acts as abinder for the phosphoric acid-resistant insulating substance such assilicon carbide according to the following formula (1):

    MO.sub.x +X·H.sub.3 PO.sub.4 →M(HPO.sub.4).sub.x +X·H.sub.2 O                                     (1)

wherein M represents a metal and X represents a number of oxygen atomsbonded with the metal.

When the metal salt such as metal chloride is used, the metal chlorideMCl_(x) reacts with phosphoric acid X/2·H₃ PO₄ to form M(HPO₄)_(x/2)which acts as the binder for silicon carbide, etc. according to thefollowing formula (2):

    MCl.sub.x +X/2·H.sub.3 PO.sub.4 →M(HPO.sub.4).sub.x/2 +X·HCl                                           (2)

When the metal such as a tetravalent metal is used, a phosphate isformed according to the following formula (3) and the phosphate acts asthe binder for silicon carbide, etc.:

    M+2H.sub.3 PO.sub.4 →M(HPO.sub.4).sub.2 +2H.sub.2   (3)

If the phosphate which acts as the binder is mixed with a substancehaving a high resistance to phosphoric acid such as silicon carbide, itbinds silicon carbide to form a firm matrix comprising silicon carbidenucleus bound by means of the phosphate.

As the metal oxide used in the reaction of above formula (1), there maybe mentioned any metal oxide reactive with phosphoric acid. As the metaloxides, zirconium oxide, titanium oxide, tin oxide, silicon oxide andaluminum oxide are suitable. These oxides may be used either alone or inthe form of a proper mixture of them. As the metal salt used in thereaction of formula (2), there may be mentioned halides, hydroxides,oxychlorides and nitrates of one or more elements selected from thegroup consisting of zirconium, titanium, tin, silicon and aluminum.

As the metal used in the reaction of above formula (3), there may beused zirconium, titanium, tin or silicon or an adequate mixture of twoor more of them. In case the oxide is used as the phosphoricacid-resistant insulating material, a part thereof may be used as thephosphate-forming substance. A care should be taken in this case, sinceif the amount of phosphoric acid is excessive, the amount of thephosphoric acid-resistant insulating substance becomes insufficient.

For the production of the electrolyte-retaining matrix from thephosphate-forming substance, two processes which will be stated beloware preferred. These two processes have advantages peculiar to them.

In both processes, the powdery phosphoric acid-resistant insulatingsubstance is mixed with the powdery phosphate-forming substance and thenphosphoric acid is added to the mixture. Then, the mixture is heated toreact the phosphate-forming substance with phosphoric acid, whereby ametal phosphate is formed which acts as the binder for the phosphoricacid-resistant insulating substance.

(i) In one of the two processes, phosphoric acid is used in an amountlarger than that required for forming the phosphate. The excessiveamount of phosphoric acid is used as the electrolyte. If the amount ofphosphoric acid added is adequate, it is unnecessary to add phosphoricacid in the step of fabricating the fuel cell. Thus, the preferredamount of phosphoric acid is the sum of the amount required for formingthe phosphate and the amount thereof used as the electrolyte.

(ii) In the other process, phosphoric acid is used in an amount requiredfor forming the phosphate. If liquid phosphoric acid is present afterthe formation of the phosphate by the reaction of the phosphate-formingsubstance with phosphoric acid, phosphoric acid absorbs the water whilethey are stored as they are. For this reason, the water must be removedbefore this system is used for the fabrication of the fuel cell. Ifphosphoric acid does not remain after the formation of the phosphate bythe reaction of the phosphate-forming substance with phosphoric acid,the water absorption is not caused during the storage. Even in casephosphoric acid is added in an amount larger than that required forforming the phosphate, the water absorption may be disregarded if theproduct is used for the fabrication of the fuel cell immediatelythereafter without the storage. The mixture must be heated for formingthe phosphate after the addition of phosphoric acid. The heatingtemperature is 100°-250° C.

If the average particle diameter of the phosphoric acid-resistantinsulating substance is smaller than 0.1 μm, the pore volume isinsufficient and, on the other hand, if it is larger than 10 μm, thecapillary cohesion power is inclined to be reduced. If the fiber size issmaller than 0.01 mm, the pore volume is insufficient and, on the otherhand, if it is larger than 0.2 mm, it becomes difficult to form thepores.

The average particle diameter of the powdery phosphate-forming substanceis preferably 0.05-5 μm, since the particles of the phosphate formed areinterspersed if the average particle diameter is larger and the bindingpower is reduced if the average particle diameter is smaller.

In the above-described process (i) for producing theelectrolyte-retaining matrix, the amount of phosphoric acid used in theform of free phosphoric acid is suitably 40-80 wt. % based on thephosphoric acid-resistant insulating substance. In the above-describedprocesses (i) and (ii), the mixture is heated to a temperature in therange of 100°-250° C. after the addition of phosphoric acid to completethe reaction and also to remove water from the resultingelectrolyte-retaining matrix. If the amount of free phosphoric acid issmaller than 40 wt. %, the pores cannot be saturated with phosphoricacid sufficiently or the viscosity of the matrix containing freephosphoric acid becomes too high to use. If said amount is larger than80 wt. %, the strength of the matrix is insufficient for the fabricationthereof on the electrode plate.

If the heating temperature after the addition of phosphoric acid islower than 100° C., the reaction cannot be completed or it becomesdifficult to remove water by-produced by the reaction. If thetemperature is higher than 250° C., phosphoric acid isdehydration-condensed and properties thereof are altered. The heatingafter the addition of phosphoric acid is effected in two stages. It ispreferred that the temperature in the latter stage is higher than thatin the former stage so as to prevent the heterogenization of the matrixdue to the rapid reaction.

An embodiment of the structure of a unit cell of the phosphoric acidtype-fuel cell of the invention is shown in FIG. 1.

Hydrogen (fuel) is fed in a hydrogen electrode 1 and oxygen (oxidizer)is fed in an air electrode 2. They are reacted electrochemically togenerate an electric energy. At the hydrogen electrode 1, hydrogen isoxidized to release hydrogen ion and electron according to the followingformula:

    H.sub.2 →2H.sup.+ +2e

Hydrogen ion and the electron are sent to the air electrode 2 throughphosphoric acid (electrolyte) and an external circuit, respectively. Atthe air electrode 2, oxygen, hydrogen ion and the electron reacttogether to form water according to the following formula:

    1/20.sub.2 +2H.sup.+ +2e→H.sub.2 O

in this case, the theoretical electric potential at 25° C. is 1.23 volt.

An electrolyte-retaining matrix 3 is interposed between the hydrogenelectrode 1 and the air electrode 2. A separator 4 is placed outside thehydrogen electrode 1 and a separator 5 is placed outside the airelectrode 2. The unit cell is thus formed. The hydrogen electrode 1comprises an electroconductive porous base 1a made of a carbon paper orthe like and a catalyst layer 1b placed thereon. The air electrode 2comprises an electroconductive porous base 2a made of a carbon paper orthe like and a catalyst layer 2b placed thereon. The separators act forseparating the gas, passing the gas and collecting the current. As thecatalyst, a noble metal such as platinum, ruthenium or palladium may beused. The matrix 3 is placed between the two electrodes by, for example,applying the matrix material to the catalyst layer. Hydrogen passes froman arrow 6a to an arrow 6b. Oxygen passes from an arrow 7a to an arrow7b.

FIG. 2 shows a model of the electrolyte-retaining matrix in the fuelcell of the present invention. Particles 3a of phosphoric acid-resistantinsulating substance are bound by a phosphate 3b formed by the reactionof the phosphate-forming substance and phosphoric acid, whereby anintended matrix is formed. Phosphoric acid 8 (electrolyte) is retainedin the pores in the matrix.

EXAMPLES Example (1)

An aqueous solution of zirconium, tin, titanium, silicon or aluminumchloride or hydroxide was added to silicon carbide powder having anaverage particle size of 0.3 μm and the mixture was stirred. Phosphoricacid was added to the mixture. The mixing ratio of silicon carbide tothe solution of zirconium, tin, titanium, silicon or aluminum salt wascontrolled so that the amount of silicon carbide would be 70 wt. % andthat of a phosphate formed by the reaction of the metal salt withphosphoric acid would be 30 wt. %. The amount of phosphoric acid wasthat required for forming the phosphate plus 60 wt. % thereof based onsilicon carbide. The mixture was heated to 200° C. for 15 hours tocomplete the reaction and also to remove water, whereby a product havinga high viscosity was obtained. The product was applied to a gasdiffusion electrode to form an electrolyte-retaining matrix. The matrixcontains 60 wt. %, based on said silicon carbide, of free phosphoricacid which acts as the electrolyte of the fuel cell. Table 1 showsviscosities of the electrolyte-retaining matrixes obtained by theprocess of the present invention and that of a phosphoric acidelectrolyte-retaining matrix obtained by a conventional process wherein5 wt. % of polytetrafluoroethylene was mixed with silicon carbide andthen 60 wt. % of phosphoric acid was added to the mixture. The viscositywas determined by applying a load of 50 kg to 5 g of the phosphoric acidelectrolyte-retaining matrix on a flat plate and measuring an area ofthe matrix thus spread. It is apparent from Table 1 that a bindingcapacity equivalent to that obtained by using polytetrafluoroethylenecan be obtained by using the inorganic binder.

A phosphoric acid-retaining capacity three times as high as that of theordinary matrix could be obtained by using the phosphoric acidelectrolyte-retaining matrix shown in the example.

                  TABLE 1                                                         ______________________________________                                        No.       Binder            Spread area                                       ______________________________________                                        1         Zirconium phosphate                                                                             50 (cm.sup.2)                                     2         Tin phosphate     45                                                3         Titanium phosphate                                                                              70                                                4         Silicon phosphate 65                                                5         Aluminum phosphate                                                                              55                                                6         Polytetrafluoroethylene                                                                         50                                                ______________________________________                                    

Example (2)

25 g of silicon carbide used in Example (1) was mixed with a solution of12.2 g of zirconium oxychloride (ZrOCl₂.8H₂ O) in 50 ml of water. Then,11 g of phosphoric acid was added to the mixture and the whole washeated at 200° C. for 15 hours to obtain an electrolyte-retainingmatrix. The matrix was applied to a known gas diffusion electrode in athickness of 0.3 mm and another gas diffusion electrode was put thereonto fabricate a fuel cell. The gas diffusion electrode had been obtainedby applying carbon powder carrying a very small amount of platinum to acarbon paper.

FIGS. 3 and 4 show a relationship between current density and voltageand a change of the cell voltage with time observed during thecontinuous use of the fuel cell containing the phosphoric acidelectrolyte-retaining matrix obtained in this example at a currentdensity of 200 mA/cm², respectively, as compared with properties of afuel cell wherein polytetrafluoroethylene was used as the binder forsilicon carbide. In FIGS. 3 and 4, the abscissae show the currentdensity (mA/cm²) and operating time (h), respectively, and the ordinatesshow voltage (V). Curves A and C represent the results obtained in thisexample and curves B and D represent those of the conventional case.

It is apparent from FIGS. 3 and 4 that when the phosphoric acidelectrolyte-retaining matrix of the example was used, the reduction ofvoltage due to the increase of current density or the reduction ofvoltage with the operating time were very slight and the capacitiesgreater than those of the conventional product could be obtained. Thisimprovement in capacity is attained because of the high affinity of thephosphoric acid compound used as the binder with phosphoric acid and thehigh retaining capacity of phosphoric acid. FIGS. 3 and 4 further showthe results obtained at an operating temperature of 190° C. It isunderstood that when the phosphoric acid electrolyte-retaining matrix ofthe example was used, stable results could be obtained even at a hightemperature.

Example (3)

3 g of zirconium oxide was added to 7 g of silicon carbide having anaverage particle diameter of 0.3 μm and they were mixed thoroughly bymeans of a mortar. Then, 9.5 g of 85% phosphoric acid was mixedtherewith. The resulting mixture was heated at 150° C. for 2 hours andthen at 200° C. for 10 hours to obtain a viscous electrolyte-retainingmatrix. About 55% of phosphoric acid mixed was used for the formation ofthe phosphate and the balance (about 45%) was used as free phosphoricacid electrolyte in the fuel cell. Ths matrix thus obtained was appliedto a gas diffusion electrode to fabricate a fuel cell havingsubstantially the same structure as that shown in FIG. 3. Theelectrolyte-retaining matrix had a thickness of about 0.3 mm. FIG. 5shows a change of the cell voltage with time observed during thecontinuous use of the fuel cell containing the electrolyte-retainingmatrix obtained in this example at an operating temperature of 190° C.and a current density of 200 mA/cm² as compared with the properties ofan ordinary fuel cell having an electrolyte-retaining matrix comprisinga mixture of silicon carbide with polytetrafluoroethylene binder. InFIG. 5, the abscissae and ordinates show operating time (h) and voltage(V), respectively and curve I shows the results of this example andcurve II shows the results obtained by using the ordinary cell. It isapparent from FIG. 5 that the deterioration of capacity was notrecognized even after the use of the fuel cell of this example for along time, while the voltage was reduced seriously with time when theordinary fuel cell was used.

Example (4)

A solution of 22 g of zirconium oxychloride (ZrOCl₂.8H₂ O) in 10 ml ofwater was added to 7 g of silicon carbide to obtain a mixture. Then, 7.5g of 85% phosphoric acid was added to the mixture. After the thoroughstirring, the mixture was heated at 150° C. for 2 hours and then at 200°C. for 10 hours to obtain a highly viscous electrolyte-retaining matrix.About 50% of phosphoric acid mixed was used for the formation of thephosphate and the balance (about 50%) was used as free phosphoric acidelectrolyte in the fuel cell. The thus obtained matrix was applied to agas diffusion electrode to fabricate a fuel cell having substantiallythe same structure as in Example (1). FIG. 6 shows a change of the cellvoltage with time observed during the operation carried out under thesame conditions as in Example (1) as compared with the properties of anordinary fuel cell having an electrolyte-retaining matrix comprising amixture of silicon carbide with polytetrafluoroethylene binder. In FIG.6, the abscissae and ordinates show operation time (h) and voltage (V),respectively and curve III shows the results of this example and curveIV shows the results obtained by using the ordinary cell. It is apparentfrom FIG. 6 that the effects obtained in Example (4) were similar tothose obtained in Example (3).

More particularly, the fuel cell containing the electrolyte-retainingmatrix of this example was stable even at a high temperature. Inaddition, the affinity of the matrix with phosphoric acid was improved,since silicon carbide was bound by the phosphate and, therefore, thepower of retaining phosphoric acid was improved remarkably and thecapacity was not deteriorated even after the operation for a long time.

We claim:
 1. A fuel cell comprising a pair of gas-diffusion electrodes and a phosphoric acid electrolyte-retaining matrix interposed between the electrodes, said matrix consisting of 30-90 wt. % of a substance unreactive with phosphoric acid and having electron-insulating properties and 10-70 wt. % of a metal phosphate binder.
 2. A fuel cell according to claim 1 wherein the metal phosphate is zirconium phosphate.
 3. A fuel cell according to claim 1 wherein the substance unreactive with phosphoric acid and having electron-insulating properties is in the form of particles having an average particle diameter of 0.1-10 μm.
 4. A fuel cell according to claim 3, wherein the matrix is a porous structure of the particles of said substance unreactive with the phosphoric acid bonded together with the metal phosphate binder.
 5. A fuel cell according to claim 1 wherein the substance unreactive with phosphoric acid and having electron-insulating properties is an inorganic substance.
 6. A fuel cell according to claim 5 wherein the inorganic substance is a carbide.
 7. A fuel cell according to claim 6 wherein the carbide is silicon carbide.
 8. A phosphoric acid-type fuel cell comprising a pair of gas-diffusion electrodes and a phosphoric acid electrolyte-retaining matrix interposed between the electrodes, said matrix consisting of 30-90 wt. % of silicon carbide particles and 10-70 wt. % of a metal phosphate binder.
 9. A phosphoric acid-type fuel cell according to claim 8, wherein said matrix is a porous structure of the silicon carbide particles bonded together with the metal phosphate binder.
 10. A phosphoric acid-type fuel cell according to claim 9, wherein the particles of silicon carbide have an average particle diameter of 0.1-10 μm. 